1. Field of the Invention
The present invention generally relates to detecting a pulse of received energy in the presence of noise. More particularly, the present invention relates to a method and apparatus for distinguishing a time-invariant pulse in a received signal, the detection of the pulse being independent of amplitude variations in the received signal and being performed without degradation in signal-to-noise ratio. The invention is applicable in systems which, for example, employ pulse transmission for time-of-flight measurements.
2. State of the Art
Pulse detection and measurement have become important in time-of-flight measurement applications such as automotive radar systems for collision avoidance. In such systems, an optical or electromagnetic pulse having a predetermined pulse shape is transmitted from an object, such as a vehicle, and used to detect the distance between that object and another object. FIG. 1 shows an example of a typical automotive radar system in which a radar device 110 is mounted on a vehicle 100. The radar device 110 emits a pulse which, as shown in FIG. 1, is transmitted to and reflected by a vehicle 120 and then received by the radar device 110. The time it takes for the pulse to travel to and from the Vehicle 120 is typically measured with a counter to provide an indication of the time of flight.
Because of the many variables which can influence the pulse between the time it is transmitted and the time it is received (such as the distance the pulse actually travels, the reflectivity of the surface which reflects the pulse, and so forth), amplitude variations from one pulse to the next are inevitable. FIG. 2 graphically depicts a problem of amplitude induced error that can occur due to amplitude variations in pulses of received energy. FIG. 2 illustrates a pulse of received energy which is detected using a counter when the pulse amplitude reaches a predetermined threshold which is set at a desired signal-to-noise ratio (SNR). FIG. 2 further illustrates how variations in amplitude influence the time at which the pulse reaches the threshold, and thus the time at which the pulse is detected by the counter. In other words, there is an amplitude induced error in the time-of-flight measurement of each pulse. The amplitude induced error represents the range in time over which pulses of differing amplitude intersect the threshold level, the exact point of intersection for each pulse being related to the pulse's amplitude.
In the past, amplitude induced errors were simply ignored because of the poor resolution of conventional counters. That is, the relatively low resolution of conventional counters was unaffected by the time variations associated with detecting the leading edge of a pulse influenced by amplitude induced errors. However, as technology has progressed, counters with higher resolutions have been developed, and the amplitude induced errors can no longer be ignored without risking pulse detection errors. Thus, various approaches have been used in attempts to improve the accuracy of pulse detection.
U.S. Pat. No. 5,243,553 discloses a pulse detection system in which a received pulse is digitized in a comparison circuit and a gate array circuit, stored in a RAM, then analyzed with an algorithm to determine a position of the pulse center. The analysis of the stored pulse involves mathematical correlation of the pulse with a template. This approach is very complex, expensive and slow, requiring storage of earlier pulse information. Additionally, the use of analog-to-digital conversion limits the bandwidth of the system, thus lowering the maximum resolution obtainable.
U.S. Pat. No. 3,906,377 discloses a second approach for pulse detection using a dual-integrating pulse centroid detector. As described therein, two integrators, a summer, and a zero crossing detector are employed to determine a centroid of a pulse. This approach requires the use of repetitive pulses to find the centroid of a pulse and is therefore impractical for use in detecting a single received pulse. Further, this approach is difficult to implement at higher bandwidths, requires the use of complex operational amplifiers for accuracy, and requires that a reset signal be asserted once a pulse has been detected (that is, subsequent to receipt of the multiple, repetitive pulses) to zero the integrators.
U.S. Pat. No. 4,495,529 discloses another approach for pulse detection on computer hard disk drives. This approach was developed to correct errors caused when a long string of zeroes is received by the read head of a hard disk drive. As described in this patent, a train of pulses are received by a read head, differentiated and their zero-crossing points determined in a zero-cross detection comparator. The train of pulses is also delivered to circuits for detecting and holding peak positive and negative voltages as reference levels for two window comparators. Signals from the zero-cross comparator and the window comparators are input to digital logic gates which compensate for a long string of zeroes. As with the approach of U.S. Pat. No. 3,906,377, this approach requires repetitive pulses and does not work at high bandwidths or with very fast pulses, (for example, bandwidths greater than 50 Megahertz (Mhz) and pulse widths less than 50 nanoseconds (ns)). This approach also requires information about previous pulses to determine the peak positive and negative voltages, which requires excessive hardware and severely limits maximum bandwidth. Further, this approach uses a digital feedback circuit to create a fixed pulse width which is not related to the actual width of the received pulse, and thus provides no pulse width information.
Other approaches also exist which are directed to detecting pulse edges rather than pulse centers. For example, U.S. Pat. No. 3,532,905 discloses constant-fraction detection of the leading edge of an incoming pulse. According to this approach, both the pulse and a delayed, attenuated version of the pulse are input to a comparator such that the leading edge is detected via a threshold which represents a percentage of the pulse's amplitude. U.S. Pat. No. 5,210,397 discloses a system in which a pulse labelled TPI is differentiated and integrated. The differential signal TP2 is compared to a delayed version of the integrated signal, resulting in a signal TP3 that transitions on the edges of the received pulse TP1. Both of these approaches are thus directed to pulse edge detection. However, while the detected edges are relatively time-invariant, the centers remain time-variant and therefore susceptible to amplitude-induced errors.
Thus, there is a need for simply and inexpensively eliminating or reducing amplitude induced timing errors in a pulse detection system, without lowering the bandwidth or the signal-to-noise ratio (SNR). In so doing, it would be desirable to eliminate any need for complex or surplus circuitry (such as circuitry that asserts an integrator reset signal between each pulse). Further, it would be desirable to provide accurate pulse detection without stripping the received signal of original pulse-width information.